Methods and apparatus for the determination of the impedance of zones of the thorax, head and limbs are known in which a current is passed through the body and potentials in the body are measured. Impedance changes may be recorded which are correlated with the cardiac, respiratory or other functions of the body due to variations in blood, air or other contents of regions of the body. For example by using such apparatus an evaluation of the stroke-volume of the heart, and respiration and perfusion of the lungs may be made.
A fundamental problem which occurs in such known methods is that a current passed into the body tends to diverge from the entry electrode until limited by an external boundary of the body, converging again at an exit electrode. With bodies having an irregular boundary suitable geometrical factors cannot be computed to allow for this boundary and changes in the boundary may be interpreted as changes in the contents of a zone measured. Further, when inhomogeneities are encountered in the body the current tends to converge towards regions of high conductivity and diverge from regions of low conductivity. Current does not therefore flow naturally in straight or even easily described curved paths in an inhomogeneous medium. Changes in the contents of a zone under investigation cause local changes in impedance and therefore in current paths so that any impedance measured changes not only because the contents of the zone change but also because the current flow patterns change.
U.S. Pat. Nos. 2,712,627 to 2,712,630 describe measuring the resistivity of earth formations by lowering electrodes into a borehole. In this method a current sheet perpendicular to the borehole is generated by injecting flanking currents but the sheet diverges so that the resistivities of volumes of material remote from the measuring point and which are irrelevant to the required value become involved in the measurement. Variants of this method are described in "The Microlaterolog" by H. G. Doll in Petroleum Transactions, AIME, Vol. 198, 1953, where the flanking current is injected by a circular electrode concentric with a primary electrode and by Jackson, Marine Geotechnology Vol. 1, no. 2, page 91 at seq. (1975) where the electrodes are grouped into two concentric sets of electrodes of opposite sign which are located on the same insulating pad. With the Microlaterolog the path of the current tube and its cross-section are thought to be modified by variations in conductivity as the electrodes are lowered below the surface; that is as the electrodes are lowered down the borehole and volume of material whose resistivity is measured varies because current patterns vary.